Electric furnace and method of operating the same



April 16, 1935.

R; C. BENNER ET AL ELECTRIC FURNACE AND METHD OF OPERATING THE SAME Filed Nov. 17, 1931 INVENTORS RAYMOND c. BENNER GEORGE J. EA$TER OLARENGE E. HAWKE mu ATTORNEW atehte Apn E& 335

PATNT g ELECTRIC FUBNACE AND METHOD OF OPERATDIG THE SADIE Raymond C. Beer and George J. Niagara Falls, N. Y., and Clarence E.

Metnchen,

Easter, Hawke,

N. J., assgnors to The Carborundum Company, Niagara Falls, N. Y., a corporation of Pennsylvania Application November l'', 1931, Serial Ne. 5 75584.-

5 Claims.

in whichJI-he ware is heated at least in part by 5 direct radiation from the resistor. The invention relates both to improvements in the Construction of such furnaces, and to a method of operation wherein intermittently inserted charges can be heated much more'effectively than has heretofore been possible. The present application is a continuation in part of our copending applications, Serial Numbers 324,418 and 3243119, both filed on December 7, 1928.

In the usual high temperature electric furnaces Operating at temperatures above 1100 C., it is not practicable to use metallic heating elements, since the common resistance alloys used for such purposes rapidly disintegrate at these high operating temperatures. Until comparatively recently the only resistor material widely used for such applications was cel-bon or graph'te. As is wellknown, carbon is unstable in air, and when used in furnaces which are to be repeatedly opened for intermittent charging the resistor is of necessity placed outside the furnace chamber and suitably protected-from oxidation. Under these conditions it is of course necessary to provide a conducting refractory to conduct the heat from the resistor into the heating chamber.

In recent years resistance elements suitable for high temperature industrial heating have been i developed in which the resistor material is silicon carbide. These reistors are comparatively stable in air, and with them it became possible for the first time to place the heating element directly i within the furnace chamber, even when the furnace was to be repeatedly opened for-the insertion or removal of the charge. With the introduction of this type of resistor it became possible to heat intermittent charges to temperatures of from 1100 to 1450 C. by direct radiation from the resistor itself.

The problem 'of efiiciently Operating a. heat treating urnace when the resistor is placed within the heating chamber is entirely different from that with the older type of furnace in which the resistor is placed outside the heating chamber. Instead of conductng heat into the chamber, the heat is generated within the chamber itself, and the problem is one of retaining the heat and preventing its passage through 'the walls of the chamber rather than conducting the heat into the chamber from an outside source. For this reason when the resistors are within the chamber the fumace walls have been constructed of refractory which is as highly insulating as is consistent with the temperatures which the meterial must withstand. It is customary to line the heating chamber of such iurnaces either with fire-clay, which is a poor thermal conductor, or With the so-called insulating refractories, in order to retain the heat in the chamber and thus increase the efficiency of the furnace.

In the operation of the usual silicon carbide resistor furnace made in this way, there are a munber of serious disadvantages, in spite of the advantages gained by placing the heating elements within the furnace chamber. During operation, only a thin inner shell of the refractory lining is heated to the Operating temperature of the fumace, and, on the insertion of a massive charge, any heat retaned in the furnace under equilibrium conditions is soon extracted. The charge must then be heated by radiation from the resistor itself. Only a portion of the charge is exposed to direct radiation, and if the power is increased to a. value sufiicient to efiect rapid heating, the parts of the charge exposed to direct radiation become overheated or burned" before heat can be conducted through the 'chargc to the more remote portions shielded from direct radiation. If the furnace is operated at a high power input and the charge removed from the fumace, the insulating nature of the furnace wall causes a rapid rise in temperature, with consequent destruction of both the resistors and the furnace lining. Uniform heating at a rapid rate with a substantially constant power input is thus impossible when the tui-hace is charged intermittently. The diificulties described have always been considered as inherent with this type of furnace.

In our improved furnace, ,we overcome these difiiculties by providing a. composite wall, a porticn of which is of highly conductng refractory so as to allow the penetration of heat into the wall of the heating chamber for a substantial depth, and the remainder of which is highly insulating so as to minimize the heat losses through the wall. We have found that if we employ for example a chamber linng of silicon carbde of substatial thickness in a urnace in which radiant resistors are contained within the heating chamber, and employ a hacking for their lining having a very poor thermal conductivity, the silicon carbide can be heated to approximately the maximum temperature of the furnace for the entire depth of lining. For example, we have found that with a silicon carbide lining four and a half inches in thickness and with an inner wall temperature of 1300 C., the thermal drop through the entire thickness of the silicon carbide is less than 20 C. when a highly insulating backing' is used, whereas the thermal drop in the insulating backing is approximately 1200" C. This penetration of heat'into the wall of the heating chamber for a considerable depth increases enormously the amount of heat available for immediate transfer to a cold charge, and the effect is the same as if the charge were completely surrounded with a high temperature resistor. The time of heating for a massive charge of metal can be decreased to approximately one tenth that when the charge is heated by radiation from the resistor alone, as is approximately the, case with a highly insulating refractory lining. The heat is applied uniformly to all parts of the charge, and particularly to the parts which would be shielded from direct radiation in the usual furnace. Although the heat losses under equilibrium conditions are somewhat increased by the introduction of a conducting refractory into the wall, we have found by actual measurement that the heat losses per pound of Ware treated are actually decreased instead of increased by the highly conducting refractory.

Some of the outstanding results effected by our composite wall structure in combination with radiant heating elements located within the heating chamber are as follows:

1. With a typical metal charge, the time of heating the charge can be decreased from ninutes to 10 minutes by replacing the insulating refractory lining with silicon carbide.

2. The *power input can be approximately doubled and the additional heat transferred to successive charges without overheating the furnace during idle periods or losing the additional heat supplied. The excess heat is retained in the urnace by absorption into the lining, and is immediately available for transfer to the cold charge. Since the amount of material which can be heated depends on the amount of useful power which can be supplied to the charge, the

' output per given furnace is greatly increased.

3. Heat is applied uniformly to all parts of the charge, including those parts shielded from direct radiation from the resistor.

4. Even when the urnace is not charged intermittently, the temperature of the lining can be kept constant over its entire surface to approximately 10 degrees centigrade, whereas with the usual lining the temperature -at various parts of the furnace fiuctuates over a wide range.

One of the chief advantages of our furnace is the retention of useful heat during idle periods, and the almost immediate transfer of this heat to the cold charge When the latter is inserted in the furnace In order to make such a method of operation effective', it is desirable to operate thefurnace at a continuously high power input, and to. proportionjthelining so that this excess heat will be absorbed into the liningduring the averagefidle period without overheatin'g the furnac'e; Many of .thesiliconparbide resistors, such as have been used heretofore for heating purposes, possess 'a negative temperature coefl'- cient' of resistanc' 'in i the range of temperatures in which the usual `furnace is oper ated, and until comparatively 'recently all silicon carbide resistors possessed thiscl'aracte'ris'tic;` -We have' found; that considerable danger *exists in using thistype of resistor with the methOd 'of operationdescribed;sinc'efthe furnace is operatin'g at 'too high' a powerin'put" for 'equilibrium conditions. If the idle period happens to be unusuaI- ly prolonged, the furnace tends to reach equilibrium and to become overheated, and owing to the decrease in resistance of the heating element, the current and hencethe power Consumption increases as the furnace temperature rises With an element having a pronounced 'negative temperature coefiicient of resistance, this sudden increase in current will cause serious damage both to the furnace and to the resistor. We have found that this diiculty can be. overcome with a special type of resistor having a positive temperature coeicient of resistance in the operating range. Recrystallzed silicon carbide resistors such as are known and sold under the trade name of Globar, and which have a positive temperature coefiicient of resistance in the higher temperature ranges of operation are now a commercial product. With such a resistor the idle period of our furnace can be prolonged much beyond the normal period without damage to the furnace, for as the temperature rises the current through the resistor decreases. Thus as the lining becomes heated to the desired maximum temperature, the power input is auto- 4 matically decreased. The heating of the highly conducting lining by means of a power input greater than that necessary to hold the furnace at the desired temperature, and decreasing the power automatically by means of the temperature coeificient of the resistor when this tem-- perature is reached, afiords an ideal method for' the continuous operation of the furnace for the intermittent heating of separate batches of material.

The detailed Construction of two specific modifications of our furnace will be evident from the accompanying drawing although it will -be realized that many structural modications can be made in the furnace without departing from the scope of our invention.

In the drawing:

Figure 1 shows a cross-section of one form of our furnace, the section being taken through one of the heating elements as indicated by the line I-I of Figure 2;

Figure 2 is a section along the line II-II of Figure 1; and

Figure 3 shows a section similar to Figure 2 of an alternative form of our furnace Construction in which the highly conductive lining is used in the lower portion only.

Referring to the drawing in more detail, a number of resistors l extend across the heating chamber 2 from side to side, electrical contact being made 'ateach end by means of a watercooled terminal 3 connected to an outside source of electricity and held against the ends of the resistor by means of suitable springs (not shown). The terminals should preferably be made of (or tipped at the hot end with) an oxidation resistant metal, such as chrome iron containing approximately 28 per cent chromium, and the contacts between the heating element l and the terminals 3 should liexwithin the wall of the furnace just outside of the chamber 2. In order to prevent short circuiting of the electric current through the brickwork of the furnace, a refractory tube 4 having high electrical resistance at elevated temperatures may be used to surround the water-cooled terminal and that portion of the' resistor which extends into the wall of the heating chamber. This tube should be a loose slip fit for the terminal 3.

The lining E ofthe furnace is'made of a 'refractory material having high thermal conductivity and high unit heat capacity, as for example, silicon carbide, and is preferably of sufllcient thickness so that its heat Capacity will be approximately as great or greater than that of any charge which will ordinarily be placed in' the urnace. It is also desirable in securing even distribution of heat that the thickness of this lining, particularly that part of it adjacent the lower part of the heating chamber, shall be from approximately one-sixth to approximately onethird of the greatest distance from any point on the inner surface of the lining to the nearest resistor.

The outer shell 'l of the furnace is made of a refractory insulatlng material having a thermal conductivity approximately one-third that of fire-clay brick or less. This lining is made of sufficient thickness to reduce the heat losses from the furnace to as great a degree as may appear practical. The outside of the iurnace may be reinforced with sheet iron or iron stays (not shown) which may also support springs used to hold the electrical connecto's 3 in contact with the resistor. Theurnaces shown in Figures i and 2 are equipped with a door provided with a handle 9 by which it may he withdrawn from its position and rested on shel i@ on the outside of the urnace while a load is being inserted or withd'awn.

In Figura 3 the highly conductive lining 8 is shown as being adjaceht the lower part of the heating chamloer only and a load such as ah alloy steel ingot M is shown in place in the himace.

In constructions of the type shown in this drawing, the resistors which we prefer to use are recrystallzed silicon carbide resistere made from special high purity silicon carbide graih prepared and burned substantially in aocord-= ance with United States Patent No. Lille-,939 of J. A. Boyer and A. J. Thompson, issue-d June 20, 1933. Resistors or this type have a very posltive temperature resistance coecient :from about 500 C. to ishi? C.

The lining Gi of the furnace is not only so reractory that it is not readily damaged but it forms a heat reservoir in which a large amount of heat can he stored at temoeratures, thus preventng' the temperature oi the inner face of the lining :from building up to a dangerous point. When the temperature of the lining 8 is low, the resistor temperature also tends to fall its resistanoe drops, so that the rate of heat input to the lining is relatively more rapid at such times and is automatically tapered oh bythe resistor as heat is stored in the lihing and as the urnaoe temperature approaches the desired In Operating our fumaces we preier to raise the temperature of the heating chamher while it is still empty to a value in excess of that to which it is desired to heat the load which is to 'se placed therein. During this period a large amount of heat is stored in the L iner lining at high ternperatures and in quickly available form. After the desirecl preheat temperatme has heeh reached, the load is inserted in the heating Cham her and heating continued while the load is `sorought to temperature. The cold load not only absorbs a large proportion of the heat given off by the resistors directly, but also withdraws a major fraction of the total heat required to raise its temperature to the desired value from the furnace lining, so that it attains the desired temperature very rapidly, as for example, in a period of a very few minutes.

The benefits resulting from our method of furnace construction can be best illustrated by describing a series of tests made with furnaces similar to that shown in Figures 1 and 2, in which the furnace dimensions were the same in all cases, and in which electrical energy was supplied at the same rate by silicon carbide resistors of the type described.

The furnaces were constructed with an inner refractory lining of thickness equal to one-fifth the width of the cubical heating chamber in each case, although a thickness of half that used would have been ample for mechanical purposes. The Volume of the lining was thus roughly 1.5 times that of the heating chamber in the furnace tested. The lining was in each case backed by a heavy layer of insulating brick of thermal conductivity about .0009 cal./cm /C./- sec. Thus, the 'only variation from furnace to furnace was in the material used for the inner lining, which was made in the various respective cases of silicon carbide, fused alumina, ordi- :nary fire-brick and finally of insulatlng refractory similar to that of the outer hacking.

Eh each case the furnace was allowed to run empty until a temperature of 1350 C. was attainecl in the heating chamber, whereupon a cold charge of alloy steel sumcient to about half fill the chamber was inserted and the door again closed while the charge was heated to l000 C.

The time required for the temperature to reach this value was found to be approximately ten minutes with the silicon carbide lining, compared with forty minutes for the fused alumina and one hour for the fire-clay lined furnace. In case of the lining of insulating material, through which the heat loss was undoubtedly conslderably reduced, the heating time was nevertheless nihety minutes.

From eonsideration of the power input to the furnace, it is estimated that the heat stored in the `iurnace linings and supplied by them to the steel in the various cases was aprcxmately as follows:

Linig SiC Ahoa Fireeay Insuleton AV. calories/min 645, 000 108, 000 48, 000 8, 000 'C-sL/miL/cm. of lining 172 29 13 2 SIC Al o Clay Insulation Conductivity (ca1./r. n.

Oleee.) 0. 0373 0. 0088 0. 0031 0, 0009 Heat cepaeity (caL/cc O. 71 0. 93 0.61 0. 26 Density (gms./cc.) i 2,48 2.81 2.05 0.87

Et will se noted that for the urnaces yielding the best results the thermal cohductivity of the urhaoe lining is in excess of 0.008 units, while the heat capacity ls g reater than 0.6 calories per oc. of reiractory.

in 'the example cited, it was found that the heat capacity of the steel charge was roughly 0.5 calorie per co. of heating chamber, and was approximately halt that of the re-clay llnings. The heat capacities of the silicon carbide and alum'na llnings, respectively, were about 2 /2 and 3 times that oi the chaue.

Other refractory materials than those specifically listed above can be utilized in the construction of our furnace linings provided their physical properties lie within the prescribed limits. Examples of other suitable refractories are fused magnesia, zircon, fused mullite, etc.

While the furnace of the type described possesses many advantages in connection with the rapid heating of intermittent charges, we find that it is also advantageous in that it produces a v much more uniform temperature on the difierent sides of a load placed in the heating chamber even under the most severe conditions which occur when both the load and the furnace are heated simultaneously from a cold start. In furnaces of this type lined with fire-clay in the customary manner, the temperature of the face of the lining has been found in many cases to vary from point to point by considerably over 100 C. It has furthermore proved practically impossible to secure satisfactorily uniform heating of the top and bottom of thick pieces of ceramic ware, for example, in this type of furnace as there is no means of conveying heat to the bottom of the ware at a rate sufiicient to mature it before the top of the ware has been seriously overheated.

With our type of furnace lining, we find that this dimculty is greatly reduced, since the radiant energy striking the lining 6 is quickly absorbed 'therein and rapidly conducted through the lining to the lower face of the load. Due to the higher conductivity of the silicon carbide lining (as compared for example with that of a six inch thick ceramic plaque under heat treatment) it is possible to deliver heat through the furnace lining to the lower face of the plaque just as readily as the part of the plaque one inch from the face directly exposed to radiation can be heated. We find that by the use of a massive lining of the type described extending continuously from a point where it is subjected to direct radiation from the heating elements to that part of the furnace which is sheltered from direct radiation by the load, it is possible to very greatly 'improve the uniformity of temperature around the load as the furnace is brought up to heat and that such a furnace is therefore applicable in many cases where a similar furnace without the massive highly conducting lining would be inapplicable.

Having thus described our invention and shown its application by exam'ples, what we claim is:

1. In a heat treating furnace, a heating chamber, silicon carbide resistors within said chamber and adjacent to the top thereof, and a contnuous massive lining of silicon carbide around the lower portion of the heating chamber, the said lining being in a fixed position with respect to the resistors and being exposed to direct radiation from the resistors except when shielded by the insertion of a charge into the furnace.

2. In a continuously operated electri'c furnace adapted for intermittent charging, a heating chamber, a silicon carbide resistor within the heating chamber, and a massive lining of highly conducting refractory having a thermal conductivity in excess of .006 ca1./cm. /sec./ C. surrounding the heating chamber, the Volume of the said lining being at least equal to the space enclosed by the heating chamber, the said lining being in a fixed position with respect to the resistor and being exposed to direct radiation from the resistor except when shielded by the insertion of a charge into the furnace.

3. The combination described in claim 2, in which the lining of highly conducting refractory is composed principally of silicon carbide.

4. In a continuously operated electric furnace adapted for intermittent charging, a heating chamber, a silicon carbide resistor within the heating chamber and adjacent one wall thereof,

adapted for intermittent charging, a heating chamber, a silicon carbide resistor within the heating chamber, the said resistor having a positive temperature coemcient of electrical resistance at its normal Operating temperature, and a massive lining of highly conducting refractory having a thermal conductivity in excess of .0067

cal./cm. /sec./ C. surrounding the said heating chamber, the said lining being in a fixed position .with respect to the resistor and being exposed to direct radiation from the resistor except when shielded by the insertion of a charge into the furnace.

RAYMOND C. BENNER. GEORGE J. EASTER. CLARENCE E. HAWKE. 

